As a method of analyzing chemical species adherent to a surface of a thin film formed on a substrate surface or the substrate surface, infrared spectroscopy has been widely used. This method is based on the fact that interatomic bonds each have a specific absorption wavelength (frequency) in an infrared region. If a sample can be filled into a cell or inserted between salt crystal plates directly or in the solution form, Fourier transform infrared spectroscopy (FTIS) is often used. In recent years, attenuated total reflection (ATR) has been employed. This method totally reflects infrared light from a transparent substrate side on a thin film formed on the transparent substrate surface to measure the degree at which the thin film absorbs evanescent waves at around an interface. The ATR has sensitivity that is about 30 times higher than permeation analysis such as FTIR.
On the other hand, cavity ring down (CRD) spectroscopy has been intensively developed in recent years. As for the cavity ring down spectroscopy, a cavity is formed using at least two mirrors, a test substance (sample) is injected into the cavity, and the sample is analyzed by spectroscopy with ring down pulse light attenuated due to light absorption of the sample in the cavity. According to the cavity ring down spectroscopy, an attenuation constant at which light intensity attenuates due to light absorption is mainly measured to thereby determine absorption coefficient of the sample for each wavelength to identify and quantitatively determine the sample. As disclosed in Patent Documents 3 and 4, known is a method of circulating pulse light in a loop fiber in place of the cavity or reciprocating pulse light in a straight fiber with the pulse light being reflected at end faces to thereby measure ring down characteristics of the pulse right.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2000-338037
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2001-194299
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2004-333337
Patent Document 4: U.S. Pat. No. 6,842,548B2
Patent Document 5: U.S. Pat. No. 7,012,696B2